In many electronic matrix systems, an array of photosensitive pixels, each capable of providing a detectable output signal, are utilized for sensing the amount of light incident thereupon. Such matrix systems typically include both linear and two dimensional arrays. Each photosensitive pixel includes photogenerative element such as a phototransistor, photoresistor or photodiode for providing a detectable signal in response to the absorption of incident light and a blocking element for selectively blocking current flow through selected portions of the pixel so as to facilitate addressing. Each pixel has a capacitance useful for storing electric charge. In the most common type of photosensitive pixel, a predetermined amount of charge is stored across the pixel capacitance and is discharged by a photocurrent produced by light incident thereupon. The charge remaining on the pixel after a predetermined period of time has elapsed is indicative of the total amount of light sensed thereby. In an alternative and less common type of pixel, the photogenerative element is a photovoltaic device such as a p-i-n diode that is operated in the fourth quadrant of its I-V curve to allow the pixel to generate its own charge in response to light incident thereupon. The total amount of charge thus generated is similarly indicative of the total amount of light sensed thereby. Either technique may be utilized to provide a data stream representing the optical information sensed by the pixel. In this manner, an array of pixels may be utilized to scan a pattern of information such as alpha-numeric information on a printed or written page, or a pattern on the surface of a workpiece and the like. Such scanners are available in a multitude of designs, including arrays made using single-crystal-silicon or gallium arsenide technologies, all thin film technologies, or hybrid technologies which combine thin film and single-crystal devices, and are well known to those of skilled in the art. Exemplary scanners made using thin film materials are described in U.S. Pat. No. 4,660,995 issued Apr. 2, 1987 and entitled "Contact-Type Document Scanner and Method", and in U.S. Pat. No. 4,675,739 issued June 23, 1987 and entitled "Integrated Radiation Sensing Array", the disclosures of which are incorporated herein by reference.
While single pixels are capable of sensing light incident thereupon and producing a corresponding signal, the utility of a single pixel imaging device is obviously limited. Typically, pixels are deployed in a linear or two dimensional array. A two dimensional array of pixels may be utilized to scan an information bearing surface in contact therewith, whereas a linear array may be moved relative to the image-bearing surface to provide a signal indicative thereof. In order to address an array of pixels, various multiplexing schemes are utilized, as detailed in an exemplary application U.S. patent application Ser. No. 885,897 filed July 15, 1986 and entitled "Signal Processing Apparatus and Method For Photosensitive Imaging System", the disclosure of which is incorporated herein by reference.
In a typical array, each pixel includes a blocking element or isolation device such as a diode, transistor or threshold switch disposed electrically in series with a photogenerative element. The blocking element assures that only electrical signals from preselected or addressed pixels are being read at any given time. When the pixel is sensing information, the photogenerative element produces an electrical signal, namely a photocurrent, which acts to dissipate the charge initially stored thereon. In the arrays using photovoltaic elements operated in the fourth quadrant, charge is cumulatively stored within the pixel. In either case, the blocking element prevents dissipation of the stored charge on the pixel. When the charge remaining or cumulatively stored in the pixel is to be read, the blocking element is rendered conductive, as for example by forward biasing a diode. The isolation element of each pixel has a capacitance associated therewith and this capacitance can degrade the operation of the pixel. For example, the capacitance of the blocking element can provide a charge of opposite polarity to that stored on the capacitance of the photogenerative element thus tending to decrease sensitivity of the pixel. This effect has been found in every multiplexing scheme heretofore utilized and is called the "capacitive kick" effect.
When the isolation device is switched from its off or blocking condition to its on or conducting condition to read the pixel by recharging the capacitance associated with the photogenerative element, the current required to dissipate the charge present on the isolation device cannot be distinguished from the current required to recharge the pixel capacitance. A similar and possibly more serious problem referred to as "capacitive kick back" occurs at the end of the read period when the isolation device is switched from its on condition to its off or blocking condition. Typically, the isolation device at this point has a significant voltage drop impressed across its current-carrying electrodes, which causes a significant transfer of charge from the capacitance of the photogenerative element of the pixel to the capacitance of the isolation element. This transfer of charge disturbs the desired voltage to be applied across the photogenerative element, and is not readily distinguishable from the current produced by reading the pixel and adversely affects the accuracy of the measurement of the light intensity incident on the pixel during the subsequent scanning cycle.
In conventional photosensitive arrays, the capacitive kick effect is always present and adversely affects the signal-to-noise ratio and accuracy of the reading obtained from the pixels. The magnitude of the capacitive kick back problem is directly related to the relative size or ratio of the capacitances of the photogenerative element and isolation element. The capacitance of circuit elements and devices is generally directly related to their physical size. For this reason, it is generally preferred in conventional designs that the relative areas of the photogenerative element to the blocking element be in a ratio of at least 5 to 1, and preferably 10 to 1 or more.
The capacitive kick becomes a significant problem severely limiting performance of high resolution photosensor arrays which include relatively small area photogenerative elements. The reason for the increase in the capacitive kick problem is that in such arrays the ratio of the areas of the photogenerative and blocking elements is significantly decreased. The desired larger ratios cannot be maintained because of constraints of lithography and processing which set a lower limit on blocking element size, particularly in large area arrays where yield problems become much more severe as minimum lithography feature sizes decrease. As a result, absolute sensitivity and speed of operation have heretofore been decreased as resolution of photosensitive pixel arrays has increased.
Obviously, it would be of great benefit to reduce or eliminate the capacitive kick effect. In accord with the principles of the instant invention, there is provided a pixel which includes a photogenerative element and a blocking means which includes at least two current blocking elements electrically interconnected in series relationship, such that the capacitive kick back effects of the blocking elements tend to cancel out. This is also believed to significantly reduce capacitive kick when the isolation means is switched from its non-conducting to conducting state. Since capacitive kick is thus significantly reduced or eliminated, sensitivity of the pixel and the accuracy with which it may be read are both significantly increased without as much regard to the relative sizes of the blocking elements and the photogenerative element. Moreover, this new pixel configuration allows the photogenerative element to be read much more quickly and with improved accuracy in comparison to conventional pixels having photogenerative and blocking elements arranged in series. The instant invention thus makes possible the fabrication of high resolution arrays of photosensitive pixels with manifest high photosensitivity and rapid read times.
These and other advantages of the instant invention will be apparent to those skilled in the art from the brief summary, the drawings, the detailed description of the drawings and the claims which follow.